Dogfood CI/docs: ensure examples use `docgen demo-function` (no downstream shim)

[cursor]

Context

courseforge/infrastructure removed tools/docgen-shim and documents docgen demo-function as the supported entry point for per-function video docs. Downstream repos (e.g. course-builder, tekton-dag) are opening parallel tasks to replace `python -m demo_function` / `PYTHONPATH=tools/docgen-shim` with the real CLI.

This repo should lead by example: workflows, README snippets, and contributor docs should invoke docgen demo-function (and the `--output` flag), not patterns that assume a vendored shim.

Suggested checks

1. Grep this repo for `demo_function`, `docgen-shim`, `python -m demo_function`.
2. Ensure `.github/workflows` (and release docs) that demonstrate per-function renders use:
   ```bash
   docgen demo-function --manifest "" --output "" [--grep ...] [--cache-dir ...]
   ```
3. Optionally add a short Contributing or CI note pointing consumers at courseforge/infrastructure `per-function-video-docs.md` § CLI for exit codes and `--manifest` shapes.

Reference

• Downstream removal / pin bump: courseforge/infrastructure#35